There has been proposed a structure in which a sensor element comprising semiconductor and a bonding portion that is electrically connected to a pad provided on the sensor element. The sensor element and the bonding portion are covered with a metal diaphragm into which oil is filled. This is disclosed in, for example, JP-A-H7-243926, which corresponds to U.S. Pat. No. 5,595,939.
In a pressure sensor having such a structure, once a fluid as a pressure medium is introduced through a pressure introduction hole provided in the pressure sensor, pressure of the fluid is applied to the sensor element via a metal diaphragm and oil. Therefore, the diaphragm provided in the sensor element is distorted, and a gage resistance formed in the diaphragm is thus transformed by compression stress or tensile stress. Thereby, a detection signal in accordance with the pressure of the fluid is outputted from the sensor element.
In the case of the pressure sensor having such a configuration, there are problems that the number of components is increased because the metal diaphragm is necessary, and a structure of the pressure sensor is complicated because a structure for sealing the oil using the metal diaphragm is necessary.
On the contrary, a structure where the metal diaphragm is eliminated can be considered. However, when such a structure is used, when the pressure sensor is used in a situation where the sensor element comprising semiconductor is exposed to a corrosive liquid, for example, the pressure sensor is used for measurement of differential pressure of DPF that is an exhaust cleaning filer of diesel-powered automobiles or pressure measurement in an atmosphere within an engine room, a problem that corrosion occurs because material of a bonding pad is Al.
Therefore, the applicants previously proposed a structure in which a bonding portion is covered with a gel protection layer, and a surface of a bonding pad comprising Al is coated by an Au plating film. Thereby, contact probability between a corrosion medium and the Al as an object to be corroded is reduced. Consequently, durability against pad corrosion can be improved. This is disclosed in Japanese Patent Application No. 2004-562923.
However, even if the above structure is used, it has been newly found that since adhesion between the protection film comprising SiN formed in the periphery of the pad and the Au film is not good, the corrosion medium that has penetrated through the gel protection layer through an interface between the Au film and the protection film enters a pad side, and therefore, the corrosion medium causes erosion.